We have developed a technique whereby it is possible to measure sodium and potassium fluxes into and out of toad urinary bladder epithelial cells. Studies have included the determination of the sites of action of ouabain and vasopressin, and of the effect of changes in potassium concentration on sodium kinetics. We plan to continue our studies on the control of sodium entry into the cells from the mucosal solutions; in particular, we are interested in the role played by calcium, pH, and anions on the entry process. In addition, we have been studying the electrical pathways in toad urinary bladder and Necturus gallbladder. In the former tissue, we have shown that there is a rapid electrical interaction between apical and basolateral borders upon addition of sodium to a sodium-free mucosal medium, or of amiloride to a sodium-containing medium. Studies are under way to seek a mechanism for this interaction, by examining the phenomenon under a variety of conditions. Our gallbladder studies involve elucidation of the control of the intracellular (shunt) conductance pathway and of the transcellular pathway. These two pathways can be separated electrically by the use of intracellular recording techniques and display different characteristics. It is possible, furthermore, to alter only a single pathway under certain conditions and hence to study it relatively independently of the other. BIBLIOGRAPHIC REFERENCES: Reuss, L., and Finn, A.L. Effects of mucosal solution hyperosmolality on the cellular and shunt pathways of Necturus gallbladder epithelium. Biophys. J., 1976, 16,7a. Reuss, L., and Finn, A.L. Mechanisms of voltage transients during trans-epithelial current clamp in Necturus gallbladder. Fed. Proc., 1976, 35,602.